George Will gets one thing right in "Dark Green Doomsayers" published in Sunday's Washington Post. Scientific predictions aren't always right. But ironically his suggestion that scientists systematically overstate the risk of environmental harm in general, and global warming in particular, was undermined by a news article published by the Post the same day reporting that many scientists now believe the predictions of the Nobel-prize-winning Intergovernmental Panel on Climate Change (IPCC) probably understate the risks we face.

Will bases his case largely on quotes from the popular press suggesting that during the 1970s scientists were predicting global cooling. The reality, as carefully documented in a recent article in the Bulletin of the American Meteorological Society, is that there was an active debate among climate scientists at the time, but most of the scientific literature pointed to global warming as a result of rising concentrations of heat-trapping carbon dioxide. Nothing like the comprehensive assessments of the IPCC existed and the science panels that looked at the emerging literature of the 1970s concluded that global warming was likely. Now we know its happening and that scientists have, if anything, been too cautious in their predictions.

The Post's news story reports on a talk at the American Association for the Advancement of Science annual meeting in which Chris Field points to data showing that global warming pollution rose faster than expected from 2000 to 2007 and evidence that global warming itself will unleash even greater quantities of heat-trapping carbon dioxide and methane from forest and tundra ecosystems, amplifying the warming effect of direct emissions. The IPCC has been cautious about including the effects of these feedback loops in their forecasts because they are difficult to quantify.

The lesson is that selective citations can be used to support almost any claim. After all, individual scientists advance the field by suggesting novel hypotheses and then testing them. Many of these hypotheses will be rejected; some will be incorporated into the scientific mainstream. But scientists as a group are cautious. So when the IPCC predicts serious consequences from global warming unless emissions are rapidly curtailed, its cold comfort indeed that a few newspapers in the 1970s speculated about possible global cooling.